Alkylhydrogenchlorosilanes are of great interest as starting materials in industrial organosilicon chemistry. By means of a combination of hydrosilylation and hydrolysis steps they open up a great variety of commercially interesting functionalized silicones.
Dimethylmonochlorosilane (DMMCS) and methyldichlorosilane are obtained as by-products of the Rochow synthesis and have to be separated from the main product dimethyldichlorosilane (DDS) (K. Schnurrbusch, Ullmanns Encylopadie der Technischem Chemie, Volume 15, Urban & Schwarzenberg-Verlag, pp. 748-769 (1964)). However, the direct synthesis which is optimized for high DDS yields is of only very limited usefulness as a source of the sought-after chlorohydrogensilanes.
A further route to dimethylmonochlorosilane is the catalytic dissociation of organochlorosilanes which are present in the residue from the direct synthesis (M. Wick, G. Kreis, F.-H. Kreuzer, Ullmanns Encylopadie der Technischen Chemie, Volume 2, 4th Edition, Verlag Chemie, pp. 485-508 (1982))).
The literature refers to many processes for converting DDS into DMMCS by means of metal hydrides. JP 89-158938 teaches a process for the partial hydrogenation of DDS using lithium hydride in an LiCl/KCl melt at from 355.degree. C. to 470.degree. C. with yields of from 8% to 17% of DMMCS being obtained. From the point of view of energy consumption, this process is not very attractive.
U.S. Pat. No. 4,115,426 describes the use of the reduction system NaH/NaBH.sub.4 in hexamethylphosphoramide (HMPA) in a temperature range from 40.degree. C. to 80.degree. C., with dimethylmonochlorosilane being formed in a yield of 71%. The extremely carcinogenic HMPA restricts broad industrial utilization of this method.
Combination of calcium hydride CaH.sub.2 and aluminum chloride or titanium hydride TiH.sub.2 and aluminum chloride have been described for the partial hydrogenation of DDS; at 250.degree. C. and 300.degree. C. in an autoclave, these give variable mixtures of DDS, DMMCS, trimethylchlorosilane and methylchlorosilane plus gaseous by-products (J. Organomet. Chem. 206 (3), pp. 279-286 (1981)). The reaction times in this high-temperature process are very long and range from 17 to 90 hours.
Hengge et al. describes the possibility of preparing partially hydrogenated organosilicon halide compounds in a system comprising trialkylstannyl chloride/sodium hydride and diethylene glycol dialkyl ethers as solvent. The reaction, which proceeds even at room temperature, requires the catalytic addition of bipyridyl or .lambda..sup.3 -phosphorus compounds and gives, e.g. when using methyltrichlorosilane, a 55% yield of methylchlorosilane plus methylsilane and methyldichlorosilane.
DE-A-44 42 753 claims alkylhydrogenchlorosilanes of the type R.sub.(4-n-m) SiCl.sub.n H.sub.m, obtainable by catalytic reaction of the corresponding alkylchlorosilanes R.sub.(4-p) SiCl.sub.p with hydrogen in the gas phase at temperatures of from 100.degree. C. to 600.degree. C. and under superatmospheric pressure, using the metals nickel and/or ruthenium and/or rhodium and/or palladium and/or platinum as catalysts, either as such or in supported form. When using DDS, this process leads to mixtures comprising dimethylmonochlorosilane, methyldichlorosilane and trimethylchlorosilane, with maximum yields of 4.0% of dimethylmonochlorosilane at very low DDS conversations (from 3.9% to 14.8) and selectivities of from 27.2% to 40.8% being achieved. To obtain isolable amounts of target product, the process therefore requires a recirculation procedure which is expensive in terms of apparatus. The selective removal of the DMMCS already formed in the reaction matrix during passage through the reactor is particularly difficult.
In view of this prior art, it is therefore an object of the present invention to develop a process for the partial hydrogenation of dimethyldichlorosilane which is an advance in terms of economics, safety and yield.
DE-C-43 13 130 teaches a process for preparing silanes or organosilicon hydrides by reduction with a magnesium hydride in a liquid reaction medium, which comprises a combination of the following features:
Use of non-pyrophoric magnesium hydride, use of customary ethers as a reaction medium, continuous removal by means of mechanical energy or ultrasound of the magnesium halide which deposits on the surface of the magnesium hydride particles during the reaction, with the preferred feature being use of a magnesium hydride which has been prepared autocatalytically.
Nonpolar reaction media such as aliphatic or cycloaliphatic hydrocarbons are not suitable for preparing silanes or organosilicon hydrides according to this characteristic process (see Comparative Example 1, experiment on the reduction of dimethyldichlorosilane).